I understand, I put the stock recovery back in place before updating.
I zeroed it out right before powering down (post ZV5-update). Then I immediately attempted to boot directly into recovery expecting to see Fastboot, but it booted the stock recovery instead.
I didn't allow the phone to boot normally after zeroing out the partition (I booted directly into recovery), so I didn't expect that /system/recovery-from-boot.p would be able to write the stock recovery back to the partition.
FWIW, I was still able to write a CWM build to the partition while booted normally and then get boot loops after trying to boot into recovery.
The next time it boots normally, the stock recovery is written back.
Perhaps the zeroing-out didn't work properly, but I think I did it correctly.
Maybe if I rename/delete /system/recovery-from-boot.p then I will still be able to access Fastboot.